1. Field of the Invention
This invention relates to a method of manufacturing a color filter for a reflective liquid crystal display, and more particularly to a method of forming a color filter on a reflective plate or reflective electrode of a reflective liquid crystal display by the use of electrodepositing paint and multilevel-development positive photoresist.
2. Description of the Prior Art
In recent years, a liquid crystal display (LCD) has been widely used in various information and communication apparatuses. Especially for portable information processors, such as notebook computers, personal digital assistants, and mobile phones, some features such as lightweight, low profile, and low power consumption have become essential requirements of portability suitable to consumers. Moreover, a reflective LCD has a quarter of the power consumption for a transmissive LCD due to an absence of a back light source and the reflective LCD has a thickness only half that of the transmissive LCD and a weight only one-third of the latter. Thus, the reflective LCD has become a key component for portable information processors.
A color filter is a key element for the LCD to present a light and lifelike picture. Even if a commercialized multicolor reflective STN-ECB LCD has been proposed (M. Ozeki, et al., SID,96, pp. 107-110), it can only present 4xcx9c6 colors and thus functions far less effectively than a reflective LCD which is integrated with a color filter and can achieve a full-color display.
The reflective LCD can be classified into two types in terms of the position of the color filter. One is located on the upper substrate and the other is located on the lower substrate. The one with the color filter, located on the upper substrate, has a structure identical to that in a general transmissive LCD. In detail, a black matrix, a color filter layer, and a protective layer are formed in sequence on a transparent substrate. However, the thickness of the color filter layer in a reflective LCD is thinner than that in a transmissive LCD, due to the fact that an incident light is passed through the color filter layer twice, so as to sustain the brightness of picture. As for the one with the color filter is, located on the lower substrate, a color filter layer is formed directly on the reflective substrate or reflective electrode, in which the existence of a black matrix is dependent upon the necessity of the display module.
The commercialized process for manufacturing a color filter generally comprises five different methods as follows: dyeing, etching, pigment dispersion, electrodeposition, and printing. In the dyeing and etching methods the dye is a main light-filtering material. The dye has advantages such as variety, chromatic uniformity, high dyeability, high chroma, and high light transmissibility, but has disadvantages such as low light-resistibility and low heat-resistibility. Thus, these two methods are gradually superseded by the pigment dispersion and the electrodeposition methods in which pigment is a main filtering material. The pigment has a better light-resistibility and heat-resistibility, and its chroma and light transmissibility can approach the grade of a dye inasmuch as the average diameter of the pigment particle is held under 0.1 xcexcm by the pigment dispersion technique.
The Pigment dispersion techniques, such as those disclosed in U.S. Pat. Nos. 5,085,973 and 4,786,148, are ones utilizing a photosensitive resin containing pigment dispensed therein and a lithophotography technique to obtain a high resolution and a freedom of arranging pixels. Such a technique can be used to manufacture a color filter for either a transmissive LCD or a reflective LCD. However, this technique has some drawbacks such as (1) low utility rate of material (1%xcx9c2%); (2) low suitability for large sizes of glass substrate; (3) several uses of an expensive high precision alignment exposure machine; (4) difficulty in controling the flatness and uniformity of the film, which in turn makes it difficult to control manufacturing costs.
Electrodeposition methods, such as disclosed in U.S. Pat. No. 4,812,387, are implemented by electrodepositing, by means of electrophoresis, the resin and pigment dispersed in water on a transparent electrode plate having a particular pattern. Thus, a light-filtering film having a good flatness and uniform thickness is obtainable. However, in such an electrodeposition method, the pattern of the electrode is limited to a stripe pattern, and thus the arrangement of pixels is limited.
As for the print method, even though it is the least expensive cost of all manufacturing methods, it is generally applied to low-end products rather than products requiring high resolution, due to the fact that it has poorest accuracy in dimension, inferior flatness, and low reliability.
In light of the advantages and drawbacks of the pigment dispersion method and the electrodeposition method, the Nippon Oil Company initially proposed a technique to manufacture the color filter for a transmissive LCD by means of incorporating the electrodeposition method with the lithophotographic method, such as disclosed in U.S. Pat. Nos. 5,214,541 and 5,214,542. The contents of the two patents are incorporated herein by reference. In detail, a mask having at least three regions of different light transmittance is used to obtain, after one exposure, several exposed areas of different exposed doses in a photoresist layer provided on a transparent conductive substrate. Then, developer solutions having different concentrations are used to remove, step by step, the several exposed areas in the photoresist layer, so as to expose a corresponding surface of the transparent conductive substrate. In the meanwhile, each time an exposed region is removed, one of the color filter layers, including a red filter, a green filter, and a blue filter, is electrodeposited on the exposed surface of the transparent conductive substrate. By means of the above method, a very fine pattern, better than that found in a conventional electrodeposition method, is obtainable, and the freedom of pattern arrangement is broadened. In addition, a filter layer having a uniform thickness and a better flatness is also obtainable. However, since an anionic electrodepositing resin is used, which is easy to be affected by a developer solution of high concentrations, the tolerance for the concentration of the developer solution is thus rather narrow. On the contrary, if a cationic electrodepositing resin is used, it is better in alkali-resistibility but will result in drawbacks such as yellowing, which in turn reduces the light transmissibility, and generates black spots on the ITO (Indium tin oxide), a transparent conductive material used as the transparent conductive substrate. Moreover, in comparison with the pigment dispersion method, an ITO layer for electrodepositing is necessary and thus will reduce light transmissibility. Therefore, the above technique still cannot supersede the pigment dispersion method.
An object of this invention is to provide a method of manufacturing a color filter for a reflective LCD by which a color filter having a large scale, fine pixels, and a better flatness is obtainable, and a large degree of freedom to arrange patterns and control of the thickness of the color filter is possible.
To achieve the above objectives, a method according to the present invention comprises the following steps: (a) coating a positive photoresist layer on a reflective substrate; (b) exposing the photoresist layer to a light via a mask having a pixel pattern such that at least three exposed regions of different exposed energies, with respect to one pixel, are formed in the photoresist layer; (c) removing one of the existing exposed regions, having the largest exposed energy by using an alkaline developer solution, so as to expose the surface of the reflective substrate corresponding to the removed region; (d) electrodepositing a paint, having a selected color on the exposed surface of the reflective substrate, so as to form a color filter layer of one selected color; and (e) repeating the steps (c) and (d) until a color filter layer consisting of all desired colors is obtained.
By means of the above method, since the color filter is directly formed on the reflective substrate or reflective electrode of the reflective LCD by an incorporation of the electrodeposition method with the lithography method, the reduction in light transmissibility due to a provision of an ITO layer is no longer considerable in comparison with the conventional pigment dispersion method. Moreover, both the anionic paint and the cationic paint are available for electrodeposition, and thus the flexibility of the positive photoresistivity in a lithography process is increased. Furthermore, merits such as a large degree of freedom in pattern arrangement, a uniform thickness, a better flatness, and a highly fine pattern are maintained.
The above advantages and features of this invention can be easily comprehended by persons skilled in the art through the drawings and detailed explanations.